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United States Patent |
5,233,497
|
Bremond
,   et al.
|
August 3, 1993
|
Overvoltage protection device
Abstract
A protection device against overvoltages liable to occur between two supply
terminals (A, B) comprises between the terminals, on the one hand, a
thyristor (Th) and, on the other hand, first and second zener diodes in
series. The anode of the first diode (D1) is connected to the cathode of
the thyristor and the cathode of the second diode (D2) is connected to the
anode of the thyristor, and a third zener diode (D3) is placed between the
thyristor gate and the junction (10) of the first and second diodes. The
third diode has an avalanche voltage higher than that of the first diode.
Inventors:
|
Bremond; Andre (Veretz, FR);
Pezzani; Robert (Vouvray, FR);
Senes; Albert (Saint Cyr sur Loire, FR)
|
Assignee:
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SGS-Thomson Microelectronics S.A. (Gentilly, FR)
|
Appl. No.:
|
728227 |
Filed:
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July 12, 1991 |
Foreign Application Priority Data
Current U.S. Class: |
361/56; 361/91.6 |
Intern'l Class: |
H02H 009/04 |
Field of Search: |
361/56,91,111
|
References Cited
U.S. Patent Documents
4280161 | Jul., 1981 | Kuhn et al. | 361/56.
|
4322767 | Mar., 1982 | El Hamamasy et al. | 361/56.
|
Primary Examiner: DeBoer; Todd E.
Attorney, Agent or Firm: Lowe, Price, LeBlanc & Becker
Claims
We claim:
1. A protection device against overvoltages liable to occur between two
supply terminals comprising, a thyristor connected between said terminals,
first and second zener diodes connected in series between said terminals,
the anode of said first diode being connected to the cathode of said
thyristor and the cathode of said second diode being connected to the
anode of said thyristor, and a third zener diode connected between the
thyristor gate and the junction of said first and second diodes, said
third diode having an avalanche voltage higher than that of said first
diode.
2. A protection circuit for protection against overvoltage surges occurring
at a pair of terminals connected to a power supply, comprising:
clipping means for limiting the voltage at said pair of terminals to a
first nominal voltage level in response to surge energy at said pair of
terminals exceeding a first predetermined threshold;
shorting means for limiting the voltage at said pair of terminals to a
second nominal voltage level, said second nominal voltage level being
substantially less than said first nominal voltage level, in response to
surge energy exceeding a second predetermined threshold higher than said
first predetermined threshold,
whereby the protection circuit automatically switches from a clipping mode
of operation to a shorting mode of operation when the surge energy reaches
said second predetermined threshold.
3. A protection circuit as recited in claim 2, wherein said clipping means
comprises first and second zener diodes connected in series across said
pair of terminals, the anode of said first zener diode being directly
connected to the cathode of said second zener diode.
4. A protection circuit as recited in claim 3, wherein said shorting means
comprises a thyristor connected in parallel across said pair of terminals,
said thyristor having a gate responsive to the voltage at the junction of
said first and second zener diodes, said clipping means thereby forming a
voltage divided input to said shorting means.
5. A protection circuit as recited in claim 4, wherein said shorting means
further comprises a third zener diode connected between the thyristor gate
and said junction, the third zener diode having an avalanche voltage
higher than the avalanche voltage of said first zener diode.
Description
BACKGROUND OF THE INVENTION
The present invention relates to an overvoltage protection device.
FIG. 1 shows the conventional arrangement of a protection device 1. Given a
power supply voltage available between the input terminals A and B, and an
electronic circuit 2 to be protected, the protection device is positioned
between the input terminals A and B.
Two main types of protection devices are commonly used.
A first type of protection device, for example a zener diode, is designed
to clip the overvoltage pulses occurring between terminals A and B. The
current/voltage characteristic of this component is of the type
illustrated in FIG. 2A, namely, as soon as the voltage across the zener
diode terminals exceeds a determined value, called breakdown voltage or
avalanche voltage V.sub.BR of the diode, the current increases up to a
substantially constant voltage. Thus, as shown in FIG. 2B, pulses P1 and
P2 that are added to a supply voltage such as a full-wave rectified
voltage are clipped and, upon the end of the pulse, the normal supply is
again present across the terminals of the device 2 to be protected.
A second type of protection device, such as a triggering avalanche
thyristor exhibits the current/voltage characteristic shown in FIG. 3A. As
soon as the voltage applied to this device exceeds a value V.sub.BO, or
break over value, the device becomes conductive and the voltage across its
terminals drops to a very low value. The system then remains at the
conductive state as long as the supply current is not decreased to a value
lower than a hold current I.sub.H. By way of example, voltage V.sub.BO can
be about a few hundreds volts and the voltage V.sub.H about ten volts. The
effect of such a protection system on a full wave rectified voltage is
illustrated in FIG. 3B. It can be noted that, from pulse P1, the device 2
is no longer supplied until the following half-period.
Each of the protection devices above described exhibits advantages and
drawbacks.
A major drawback of zener diode systems is that, when pulses have a long
duration time, a substantially high current flows during the pulse
duration though the diode which has a high voltage across its terminals
(about 400volts, for example, for a mains protection device) which causes
an increase of the diode temperature . It is then necessary to provide
large size and costly diodes.
A major drawback of devices of the avalanche thyristor type is that, after
each overvoltage, supply is interrupted until resetting of the supply
voltage. Thereby, malfunctions occur in the device to be protected that is
no longer energized or that has to include a high input tank capacitor to
palliate these voltage drops. Despite this drawback, one is induced to use
protection devices of this type when overvoltages are liable to be of high
energy (large amplitude or duration).
However, in practice, the problem encountered is somewhat different.
Indeed, FIG. 4 shows the results of a statistic survey achieved on
subscribers' lines in Europe. This survey corresponds to an observation
for 112 days of a subscriber's line and shows the occurrence of 1009
overvoltages. More particularly, FIG. 4 is a table showing the probability
of occurrence of overvoltages of determined amplitude and duration. The
table of FIG. 4 shows that 29.44% of the observed overvoltages have a
value ranging from 200 to 300 volts above the normal mains voltage and a
duration ranging from 1 to 3 microseconds whereas 0.42% only of the
observed overvoltages have a value ranging from 600 to 700 volts and a
duration ranging from 3 to 10 microseconds.
The observed overvoltages can be classified into two groups: high energy
and low energy overvoltages. High energy overvoltages are characterized
either by a long time duration (for example over 10 microseconds) even if
their amplitude is relatively low (for example lower than 300 volts), or
by a high amplitude (for example over 600 volts) even if their duration is
relatively short (for example smaller than one microsecond). Low energy
overvoltages exhibit complementary characteristics. In the above example,
they have an amplitude lower than 600 volts and a time duration smaller
than 10 microseconds.
Referring to the table of FIG. 4, it can be noted that low energy pulses
occur in 96.38% of the observed cases, whereas high current overvoltages
represent only 3.62% of the cases. However, conventionally, and to take
into account high energy overvoltages, a clipping-type protection device
such as a zener diode is not sufficient and it is necessary to resort to a
shorting-type device such as avalanche thyristor.
SUMMARY OF THE INVENTION
An object of the invention is to provide a protection device operating in a
clipping mode for low energy overvoltage pulses and in a shorting mode
only for high energy pulses.
To attain these objects and others, the invention provides a protection
device against overvoltages liable to occur between two supply terminals,
comprising, between these terminals, on the one hand, a thyristor and, on
the other hand, first and second zener diodes in series, the anode of the
first diode being connected to the cathode of the thyristor and the
cathode of the second diode being connected to the anode of the thyristor,
and a third zener diode positioned between the thyristor gate and the
junction of the first and second diodes, the third diode having an
avalanche voltage higher than that of the first diode.
BRIEF DESCRIPTION OF THE DRAWINGS
The foregoing and other objects, features and advantages of the invention
will be apparent from the following detailed description of preferred
embodiments as illustrated in the accompanying FIGS. wherein:
FIGS. 1-4, designed to illustrate the prior art and the problem that the
invention aims at solving, are above described;
FIG. 5 is a circuit diagram of a protection device according to the
invention; and
FIGS. 6A and 6B show two specific types of operation of the protection
device according to the invention.
DETAILED DESCRIPTION OF THE INVENTION
As shown in FIG. 5, the device according to the invention comprises,
between terminals A and B wherein is liable to occur an overvoltage pulse,
the series connection of two diodes D2 and D1, the cathode of diode D2
being connected to the terminal A and the anode of diode D1 to terminal B.
Between terminals A and B, is also positioned a thyristor Th, the anode of
which is connected to terminal A and the cathode of which is connected to
terminal B. The thyristor comprises a cathode gate connected through a
third zener diode D3 to the junction 10 of diodes D1 and D2. Thyristor Th
is selected so that its break over voltage V.sub.BO is substantially
higher than the sum of the avalanche voltages of diodes D1 and D2. Thus,
in the invention, thyristor Th can be rendered conductive only by gate
control.
Diodes D1, D2 and D3 have avalanche voltages equal to V.sub.BR1, V.sub.BR2
and V.sub.BR3, respectively. According to the invention, V.sub.BR3 is
selected higher than V.sub.BR1. Given the voltage values liable to be
applied to the protection devices (about a few hundreds volts), the
gate-cathode voltage drop of the thyristor (about 1 volt) is negligible
and will not be considered in the subsequent explanations.
FIG. 6A is a current/voltage (I/V) diagram of the operation of the
protection device according to the invention for low energy overvoltages.
When the overvoltage value becomes higher than V.sub.BR1 +V.sub.BR2, both
diodes D2 and D1 are set to the avalanche mode and maintain the voltage
across their terminals substantially to respective values V.sub.BR1 and
VBR2. However, as shown by curves 21 and 22 drawn in dot-and-dash line,
the voltage across their terminals slightly increases while a current
circulates therethrough. The resulting V.sub.AB voltage is represented by
a curve drawn in solid line.
FIG. 6B shows the same current/voltage diagram in case of a high current or
long overvoltage. Once the diodes are set to the avalanche mode, a
relatively high current circulates and, if the overvoltage has a high
amplitude or a long time duration, the voltage across the terminals of
each diode is increased especially because of temperature rise, as shown
by the curves 31 and 32 drawn in dotted line, the resulting voltage
V.sub.AB being illustrated by the curve 33. As soon as the voltage across
diode D1 reaches the avalanche voltage V.sub.BR3 of diode D3, thyristor Th
is set conductive and the resulting voltage V.sub.AB follows the path
34-35. Then, the thyristor remains conductive until the current becomes
lower than the hold current I.sub.H of the thyristor.
Then, a device operating as a clipping circuit for low current pulses and
as a shorting circuit for high current pulses is obtained.
Those skilled in the art will be able, as a function of the characteristics
they desire to obtain for the protection device in view of a specific
application, to select the difference between the avalanche voltages of
diodes D1 and D3 which determines the transition from a clipping device to
a shorting device operation.
On the other hand, it will be noted that the slopes and shapes of the
current/voltage diagrams are arbitrarily drawn in order to simplify the
illustration and the disclosure of the invention. Those skilled in the art
will be able to refer to the usual characteristics of zener diodes and
thyristors to draw accurate diagrams.
In practice, the sum of the avalanche voltages of diodes D1 and D2 can be
chosen close to 400 volts, both diodes being identical, and diode D3 can
be chosen with an avalanche voltage of about 250 volts. In such a way, the
system acts as a clipping circuit substantially from 400 to 450 volts,
then as a shorting circuit over 450 volts.
The invention has been disclosed in a general way and in connection with
preferred embodiments. Those skilled in the art will be able to bring
various variants and modifications. Especially, they will be able to use
components other than zener diode and thyristors but having similar
functions. On the other hand, an unidirectional protection device is
described above. The invention similarly applies to bidirectional
protection components; the changes and connections designed to form
bidirectional protection devices from unidirectional protection devices
being well known by those skilled in the art.
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